So how can you use resistors for you robot? Well LED's require resistors in series or
they burn out. Voltage Amplifiers will need them.
Almost every circuit you build will require them for some particular reason. Even if you
do not put a resistor in the circuit, theoretically your circuit will still have resistance
as all wires carry resistance.

If a transistor is turned on, current will flow from the Collector to the Emitter.
If the N-channel MOSFET is turned on, current will flow from the Drain to the Source.
Yes, the names are counter intuitive.
What makes transistors more complex is that Collector to Emitter current is not just dependent on Base voltage but also
on Base current. All quite complicated . . . then you got Darlington Pairs which are just crazy and stuff . . .
MOSFET's are just simple on and off. There is however MOSFETS designed for various applications. But usually a PWM
optimized MOSFET would be what you want.

Another note on MOSFETS, they are more efficient with higher Gate voltages. So you probably do not want to just apply
a simple binary 5V. Instead you should amplify it with say an operational amplifier or even better, a
MOSFET Driver IC. All are quite cheap and easy to implement. I highly recommend amplifying the Gate voltage
because it would save on battery power, and significantly reduce overheating. Which reminds me of another point . . .

DONT FORGET TO PUT A HEAT SINK ON ALL POWER MOSFETS!!!!!!!
They heat up pretty quick and burn you when you touch them.
A typical MOSFET is rated for about 300F, but its quite easy for them to reach that temperature.

Voltage Regulator IC's have various uses, but are usually used for feeding 5V
to anything which uses digital logic. Voltage regulators can be easily bought
as an IC (check ads on right side of this page), so you do not need to make one
on your own . . . unless of course you have
crazy power or voltage requirements - such as for robots with ray guns for eyes.

There are some general guidelines to follow when using voltage regulators.
Voltage regulators will bring any voltage to the set voltage, assuming enough current is available.
A 20V battery can also be regulated down to 5V too. However to drop a voltage, current must
be expended as heat. Inneffecient! Always try to have the smallest voltage change possible when
using regulators. If you need a regulated 5V, use a 6V NiMH pack for example.